There are a few different equations to determine port length. They can be found on numerous sites. I use the following.
This one comes from a loudspeaker cookbook I read once upon a time. It also can be found online at numerous sites.

Here we go. The box I want is 6 cubic ft. tuned to 40hz, so lets fill in what we already know. I'll be using a square vent.

Lv= {(14630000 x R²) / (Fb²) x [(Vb/Np) x 1728]} - 1.463 x R

Lv=?
Fb=40
R=?Vb=6

Lets solve for R. This is a square so we use the asterisk for a square vent. The vent of my box is going to be 15.5" x 8", (these
numbers are up to you, multiplying these numbers together determines your port area in inches, which is a number we don't need
but will help in deciding how big to make the port).

Now to figure the box to acquire this port. First let's find how much air displacement this port has. We're assuming the box
has the port in the middle so we add wood thickness to the port width twice because of the walls found inside the box, the
top and bottom of the port are part of the outer walls resulting in no displacement for those. This will change depending
on how your port is. You should be able to figure out where you should and shouldn't need to add wood thickness in the
following equation. We also assume we're using 3/4" mdf for the wood.

Re: How To: Design a ported box using pencil & paper

I did very well in calculus thank you very much...

but I seriously just **** myself. Unless you can write a source code for that and pack it into a nice little program for people to use, then I just can't see it being sought after by very many people. I'm not doubting it's accuracy, I'm just saying it's not exactly "ca.com user friendly". People around here like instant gratification

im calling goob greedy because the ad revenue was suppost to go to site maintenance and upkeep, i havent seen anything but a slow *** server, a virus, self centered mods, and excuses to be expected from steve meade. I just hope the money is going to a good cause such as a heroin addiction

Re: How To: Design a ported box using pencil & paper

Originally Posted by DNick454

I did very well in calculus thank you very much...

but I seriously just **** myself. Unless you can write a source code for that and pack it into a nice little program for people to use, then I just can't see it being sought after by very many people. I'm not doubting it's accuracy, I'm just saying it's not exactly "ca.com user friendly". People around here like instant gratification

I'll look into writing a program when I get home. Depends whether or not I can remember how And if I still have all my programs etc.